He was supposed to be caring for his sister and two cousins. Instead he killed them, he told police.

Prince George’s County police arrested Antonio Williams, 25, in the murder of three young girls found dead in Clinton, Md., Aug. 18. Williams has been arrested on three counts of first-degree murder and other related charges. (WUSA9)

Antonio Williams was supposed to be caring for his 6-year-old sister and two young cousins as his mother worked an overnight shift as a nurse. Instead he methodically killed each girl as they lay in a single bed in a Maryland home, Prince George’s County police said.

The mother, Andrena Kelley, returned to the Clinton residence early Friday morning to find the gruesome scene, the bodies of her daughter, Nadira Janae’ Withers, 6; and visiting cousins, Ariana Elizabeth DeCree, 9; and Ajayah Royale DeCree, 6, brutally stabbed in a basement bedroom.

Williams, 25, made no attempt to flee, later confessing what he had done, police said at a Saturday news conference offering details about the crime. But as of Saturday, police said there was still a mystery at the heart of the case: What prompted Williams, who marked his birthday Saturday, to carry out such horrific slayings?

Investigators said they were still probing for a motive.

Williams spared the life of his 2-year-old sister, who was found unharmed in the home, police said.

The killings shook even hardened officers and first responders, Assistant Prince George’s County Police Chief Hector Velez said at a Saturday news conference. Counselors were made available to them.

“Incidents like these are not easy to handle,” Velez said. “As parents, aunts, uncles and siblings of young children, it hurts us as well. Our initial thought is to want to hug our children, but we know we have to put those feelings aside and remain steadfast in finding who the killer is and telling the story that these three children cannot tell.”

That story began shortly around 7:30 a.m. Friday, when Kelley returned to the gray single-family home in a quiet neighborhood. She called 911 to report finding the girls unresponsive.

Investigators are still trying to determine when exactly the stabbings took place, and autopsies are being performed on the bodies.

“All three girls were found lying in a single bed suffering from puncture wounds and lacerations,” said Prince George’s County Police Capt. Anthony Schartner at the news conference. He added later: “Mr. Williams was the only adult at the time in the home.”

A Prince George’s County police car blocks Brooke Jane Drive, where three young girls were slain in Clinton, Md. (Bonnie Jo Mount/Washington Post)

Police said the DeCree sisters were visiting for the summer from Newark. Their mother and Nadira’s mother are first cousins.

Police said they had not had contact with Williams before the incident and had never been called to the address in the 6400 block of Brooke Jane Drive, where the girls were found dead. Williams has been charged with three counts of first-degree murder and other related charges.

Relatives of the girls declined to comment or did not respond to requests for comment.

A woman who answered a call to a number connected to Withers’s father said they were not ready to speak about the loss of the children.

“Right now, we’re very distraught,” she said.

On Saturday, a note addressed to the media sat outside the door of an address associated with the Withers family, asking for time to grieve.

“Our hearts are extremely heavy during this time,” read the note signed “The Withers Family.”

An online wedding page showed Nadira Withers had served as a flower girl at her father’s wedding last summer. In a photo, she’s smiling, her hair styled in a bob, and her hand resting across her heart.

The mother of the DeCree girls did not respond to a request for comment, but a video message that appeared to be from her was posted on the woman’s Facebook page on Saturday.

“Hug you all’s loved ones tonight,” the woman said in the video. “Hold the people you hold close tonight because I never thought I would really say this and mean this. Tomorrow is not guaranteed. It’s not guaranteed for you. It’s not guaranteed for me, so if we leave with a message of just love who you love. Let go of those . . . family grudges. Let go of those friend grudges. Just let go . . . because your family may not be here tomorrow.”

On Saturday, red crime scene tape still wrapped around the carport of the brick home where the girls were killed.

Pink Barbie curtains draped a window inside the home, which remained sealed under police custody.

The shock of the crimes continued to rattle the community.

Camri Thomas, 23, arrived at the home on Saturday afternoon to drop off flowers at the growing memorial of pink and yellow bouquets at the family’s mailbox.

Thomas said that she didn’t know the family but that Clinton is a tightknit community horrified at the violence that cut short the lives of three promising little girls.

“For three young girls to be murdered like that is heartbreaking,” Thomas said. “We have been thinking about it all day. We haven’t been sleeping.”